The diamond layers of PDC drill bit cutters are extremely wear and abrasion resistant but can readily suffer chipping when exposed to impact or high point loading during shipping, handling, and running into the wellbore. The cutters are also susceptible to diamond graphitization at the cutting tip due to a chemical reaction with ferrous materials at high frictional temperatures produced during cutting when ferrous materials are encountered, such as in the drilling out of casing windows or the drilling out of casing-associated equipment. Other materials, such as tungsten carbide, or cubic boron nitride (CBN), are better at cutting ferrous materials but are not as effective at cutting rock that is encountered for instance after casing or casing-associated components have been drilled through. For the purposes of this disclosure, “casing-associated component” is meant to include, but is not limited to, the following: stage cementing equipment, float shoes, shoe tracks, float collars, float valves, wipers, activation darts, activation balls, inflatable packers, mechanical packers, swellable packers, circulation subs, casing shoes, casing bits, reamer shoes, guide reamers, liner guides, liner bits, motor driven shoes, motor driven reamers, motor driven bits, disposable or one-trip motors, and disposable or one-trip turbines. In other words, a “casing-associated component” is defined as any deployed or installed obstruction within a well bore casing, or mounted within, at, or outside the end of the casing, that may be encountered in whole or in part by a drill bit.
Historically, ferrous materials associated with casing-associated components were drilled out with a specialty bit or milling tool before the preferred bit for the formation application was tripped into the hole. The potential cost savings in trip time of having a bit that could effectively drill through the casing or casing-associated equipment drove the development of new combination bits oftentimes referred to as mill drills. Bits in this area of art are typically called upon to drill between 1 and 35 linear feet of casing or casing-associated components. In the instance of casing window milling the tools must remove a few lateral inches of casing wall thickness while drilling down several linear feet. In casing exit milling, the distance to be drilled through the casing wall is dependent on the configuration and slope angle of the whipstock that is used to push the bit into the casing wall. In both cases, the relatively short amount of drilling of the casing or casing-associated equipment occurs prior to being called upon to drill hundreds or even several thousands of feet of formation.
Prior art efforts to provide for solutions to cutter protection and/or casing and casing-associated component milling, and subsequent formation drilling are set forth below. All references discussed herein are incorporated by reference.
U.S. Pat. No. 4,397,361 to Langford describes abradable cutter protection afforded by individual protrusions projecting from the head portion of the bit more than the extension of the PDC cutting elements. These protrusions are fabricated of a metal more readily abraded by the earth formation than any of the cutting elements.
U.S. Pat. Nos. 4,995,887 and 5,025,874 to Barr et al describe PDC cutters which have an additional layer of tungsten carbide bonded to the face of the diamond layer. This bonding is achieved in a high temperature, high pressure press. What is described are “cutting elements in which a further front layer of less hard material, usually again tungsten carbide, is bonded to the front face of the diamond layer and extends across at least the major part thereof. Since the less hard material of the further layer may have better toughness in tension than the diamond layer, this may enable the cutting element better to resist tensile stress. . . . ” The drawbacks of this approach are discussed herein under.
U.S. Pat. No. 5,979,571 to Scott et al describes a “Combination Milling Tool and Drill Bit”. In the Scott approach, tungsten carbide inserts are mounted in an outward row on a blade that extends from the main body of the drill bit. The outward mounted tungsten carbide inserts attached to the outward projecting portion of a blade are meant to protect an underlying row of PDC inserts connected to the same blade. Alternatively, a more outwardly projecting blade carrying tungsten carbide inserts acts to protect a less outwardly projecting blade carrying PDC inserts. In either case, the parent blade material of the combined blade or of the separate blades will create a bearing area after the tungsten carbide cutters have worn away. In another embodiment, a tungsten carbide layer is pressed in a high pressure/high temperature press onto the face of the PDC cutters. The drawbacks of this approach are discussed herein under. In another embodiment PDC cutters are embedded in the center of a ring of protective tungsten carbide insert material. In the case where the cutters are embedded in a ring of tungsten carbide the face of the PDC portion of the cutters is fully exposed and unprotected from metal debris encountered during drill out. In addition, as the combined element enters formation and the tungsten carbide ring begins to wear, bearing areas of tungsten carbide co-exist with and are adjacent to the PDC diamond layer throughout the life of the bit. In addition, the surrounding rings of tungsten carbide either reduce the total number of cutters that can be placed on a blade or overall bit face, or they reduce the diameter of the PDC diamond layers available for formation cutting. Either of these choices represents compromising departures from standard PDC bit designs.
U.S. Pat. No. 5,887,668 to Haugen et al describes milling bits with a sacrificial nose cone beneath the bit, a cutting structure intended to mill a window, and in some embodiments a cutting structure intended to drill ahead in formation. The bits described by Haugen are purpose built for these operations.
U.S. Pat. No. 6,612,383 to Desai et al describes a dual function drag bit using PDC cutters faced with a bonded tungsten carbide layer. These cutters are described as being made in a high temperature/high pressure press. The drawbacks of this approach are discussed herein under.
U.S. Pat. No. 7,178,609 to Hart et al describes a Window Mill and Drill Bit that uses separate blades or cutter sets of primary cutting structure for milling and secondary blades or cutter sets for formation drilling. In addition, Hart describes an attachment method whereby the Mill is attached to a whipstock boss using a shear bolt that directly attaches to a threaded socket deployed in a purpose built relief area on the working face of the mill.
U.S. Patent Application Publication No. 2006/0070771 to McClain et al describes Earth Boring Drill Bits with Casing Component Drill Out Capability and Methods of Use. Cutting elements aimed at cutting through wellbore equipment are deployed in separate, more highly exposed sets than cutters aimed at drilling the formation.
U.S. Patent Application Publication No. 2007/0079995 to McClain et al describes Cutting Elements Configured for Casing Component Drillout and Earth Boring Drill Bits Including Same. FIGS. 7A and 7B of the '995 application show a bonded cutter wherein the leading superabrasive element is bonded to a backing abrasive element that protrudes beyond the top of the circular, leading superabrasive element.
U.S. Patent Application Publication No. 2008/0308276 to Scott points out that “One drawback associated with providing two sets of cutting elements on a drill bit . . . is an inability to provide an optimum cutting element layout for drilling the formation after penetration of casing or casing components and surrounding cement. This issue manifests itself not only in problems with attaining an optimum cutting action, but also in problems, due to the presence of the required two sets of cutting elements, with implementing a bit hydraulics scheme effective to clear formation cuttings using a drilling fluid when any substantial rate of penetration (ROP) is sought.” Scott's solution to the drawback is to provide the drill bit with cutters configured (via coating, deposition, or HPHT bonding) with a non-reactive superabrasive material, such as cubic boron nitride, overlaying or deployed with traditional diamond cutting material, such as PDC. In other words the solution requires specialized, non-traditional PDC cutters. The solution cannot be retrofitted into a standard PDC bit but rather must be substituted for standard PDC cutters.
To broadly summarize, the solutions proposed in the prior art in this area fall into two categories: 1) Make an additional standalone structure (including separate sockets, mounting, blades, and or pre-bonded elements) of overexposed metal (typically tungsten carbide) elements to protect the primary PDC cutters in an axial direction and/or accomplish the initial milling task. In these instances the superabrasive elements can be removed from the bit and a standalone cutting structure will remain. 2) Make special PDC cutters faced with bonded (typically via HPHT methods) tungsten carbide or other non-diamond material that can accomplish the milling task prior to the traditional diamond, typically PDC, coming into play to cut the formation.
These solutions need to be evaluated in light of the body of knowledge that exists in the PDC drill bit art. Some key points are as follows:                It has been demonstrated that even slight rounding of the PDC cutter edges can reduce rate of penetration in a significant and adverse manner in many formations.        It has been demonstrated that PDC is superior to tungsten carbide, and cubic boron nitride (CBN) or other superabrasive materials for formation drilling.        It has been demonstrated that inefficient cleaning and cooling of the bit adversely affects penetration rate and bit life.        It has been demonstrated that non-formation cutting elements that come in contact with the rock face generate heat and limit penetration rate by setting up non-cutting load bearing areas of the bit face.        It has been demonstrated that force balanced PDC drill bits last longer and perform better than non-force balanced PDC drill bits.        It has been demonstrated that any type of thermal insulation of the cutting tip can accelerate the wear rate and thermal deterioration of the PDC diamond. Reference is made to SPE 16699 Sinor and Warren “Drag Bit Wear Model” and SPE11947 Glowka and Stone “Thermal Response of Polycrystalline Diamond Compact Cutters Under Simulated Downhole Conditions” the disclosures of which are incorporated by reference in their entirety. It follows that a layer of tungsten carbide, or any other material with a lower thermal conductivity than diamond that is pressed onto the face of a PDC diamond layer, will act as a thermal blanket throughout the life of the outer layer which in all likelihood will match the useful life of the diamond layer.        It has been demonstrated that the HPHT process of bonding diamond and tungsten carbide leaves residual stresses at the interface.        It has been demonstrated that cracking caused by impact, or by residual stresses in the bonded tungsten carbide can propagate into the diamond layer leading to macro chipping and failure of the diamond tip.        It is known that backrakes in the range of 10° to 25° are best for attacking rock formations while backrakes of 2° to 7° are best for machining metals. It follows that cutters wherein a planar tungsten carbide layer, or other material has been flatly pressed against the diamond layer of a PDC cutter will by definition have the same backrake angle as the underlying cutter. When deployed on a mill drill tool these cutters will by definition have backrakes that are non-optimized for either metal machining or rock cutting.        It has been demonstrated that even when a casing shoe bit that is primarily constructed of non-ferrous material is drilled out the tungsten carbide substrates of the PDC cutters deployed on the casing shoe bit can damage the PDC cutters of the bit being used to accomplish the drill out. This can occur even if an overexposed cutting structure of tungsten carbide is deployed on the drill out bit because the freed PDC cutters of the casing shoe bit can roll around underneath the drill out bit and can readily impact and damage the PDC cutters of the drill out bit by impacting the face of the PDC cutters.        It has been demonstrated that all drill out applications including float equipment, shoe tracks, casing shoes, casing reamers, casing bits, stage cementing equipment, one-trip or disposable motors or turbines, or exit windows may have damaging effects on standard PDC bits. This continues to be the case even when great efforts are made in design and material substitutions to make the equipment more drill out friendly. The use of aluminum, phenolic, and other material has been helpful in limiting PDC bit damage but has left open the possibility of damage that can reduce the performance and useful life of a PDC bit in the drilling of formation after the drill out has occurred.        
Evaluating the key points provided above demonstrates that the solutions of the prior art discussed above all embody significant design or construction compromises that substantially reduce the potential performance of the drill bit in the drilling of the formation, where it is going to spend the vast majority of its life whether measured in rotating hours or distance drilled. The prior art solutions require invasive modifications to the bit's design layout or the substitution of specialty cutters that are by definition non-optimized for formation cutting.
What is needed is a solution that allows for the use of standard PDC cutters, and formation optimized PDC bit designs without creating long-lived bearing areas. The solution should be capable of readily being retrofitted onto existing drill bits or drill bit designs and offer substantial cutter tip and cutter face protection, effective and rapid milling, and predictable and complete detachment from the bit or the cutters of the bit early in the course of the post casing/casing-associated equipment milling and drilling.